1. Field of the Invention
This invention relates to forming thin crystalline layers of films. In one of its aspects, the invention relates to a method for forming a crystalline film of, for example, a semiconductor material. In another of its aspects, the invention relates to a method for forming a semiconductor device. In still another of its aspects, the invention relates to an apparatus for forming crystalline layers, predominantly single crystalline in nature. In still another of its aspects, the invention relates to semiconductor devices.
2. State of the Prior Art
Semiconductor devices having semiconductor layers bonded to a conductive substrate are used for many electrical purposes including the generation of electricity from light. Such devices are commonly known as solar cells of photovoltaic cells. Such devices are currently manufactured by first providing a single-crystal rod by, for example, the so-called Czochralski method wherein the rod is pulled from a mass of molten raw material. The rod is thereafter ground to achieve a given outer diameter and the rod is cut in thin slices transversely to the axis of the rod to provide thin semiconductor layers, thus resulting in significant waste of material due to the thickness of the saw blade.
The layers thus cut are lapped to produce a smooth surface which is then doped with an impurity to form a junction. A grid is thereafter plated onto the smooth surface and a conductive substrate is plated or bonded onto the obverse side thereof. Conductive leads are then secured to both sides of the device.
The above system is time consuming, intensive of highly skilled labor and highly inefficient in terms of the use of the single crystalline material. Thus, devices produced by the crystal-pulling method tend to be quite expensive. As a result, the solar cell devices produced by this method are presently not competitive with conventional forms of electricity generation.
Thin films of semiconductor materials can be grown by vapor deposition onto substrates. However, it is believed that the substrates must be single crystals with a lattice structure similar to the film layer in order for the deposited film to be a single crystal, thereby limiting the applications of such methods.
Single crystals have also been formed in a boat or container which is progressively passed through a heat zone to render molten the contained material. Progressive cooling thereafter solidifies the material into one or more single crystals. This method however cannot practically be used for thin layers of, for example, silicon, because of the high surface tension of such material at the melting point thereof. Furthermore, deposition of silicon on carbon substrates has been accompanied by the formation of silicon carbide which is undesirable in solar cell devices because of the insulating properties of the same.